In the electrophotography, the copying speed is an important problem. Increase of the copying speed can be tentatively attained if the copying machine is designed so that the copying speed of the machine per se is high. However, this alone is insufficient for attaining high-speed reproduction while maintaining a good quality of a copied image. Namely, for this purpose, it is necessary to improve the properties of developer materials, especially a toner.
However, conventional toners are not sufficiently satisfactory as toners for high-speed reproduction. The reasons are as follows. Namely, since the quantity of heat received by toner particles on a copying sheet from a heat-fixing roll at high-speed reproduction is smaller than the quantity of heat received at low-speed reproduction and the speed at which the copying sheet deprives the heat-fixing roll of heat is increased, the surface temperature of the heat-fixing roll is abruptly lowered and fixation of the toner is degraded. Accordingly, it is required that fixation can be accomplished with a smaller quantity of heat and an offset phenomenon should not be caused at the fixing temperature and fixing speed. However, this requirement cannot be satisfied by conventional toners comprising a binder resin composed mainly of styrene and carbon black.
As means for solving this problem, there can be mentioned, for example, a method in which a fixing roll is coated with a silicone oil or the like to prevent occurrence of the offset phenomenon. However, according to this method, if an offset phenomenon-preventing liquid is not supplied at certain time intervals, the offset phenomenon is gradually caused and finally, a complete offset phenomenon is caused. Accordingly, in order to prevent reduction of the image quality, a silicone oil or the like should be frequently supplied, and a great effort is necessary for so-called maintenance and interior of a copying machine is contaminated with a thermal deterioration product of the oil. This problem is very serious from the practical viewpoint.
Japanese Patent Application Laid-Open Specification No. 101031/74 discloses a method in which the offset resistance is improved by partially crosslinking a binder resin. In this method, the crosslinking reaction by a vinyl monomer is a chain reaction by a radical reaction and control of this reaction is very difficult. Although occurrence of the offset phenomenon at high temperatures can be prevented to some extent, since the lowest fixation temperature is simultaneously elevated, fixation with a small quantity of heat becomes difficult, and therefore, in order to attain a high copying speed, it is indispensable to set the fixation temperature at a high level. However, elevation of the fixing temperature brings about various troubles. For example, the electric capacity of the copying machine cannot be increased and deterioration of a copying sheet is caused. Accordingly, high-speed reproduction by this method is difficult.
Furthermore, Japanese Patent Application Laid-Open Specification No. 50448/84 discloses a toner comprising a resin of a copolymer of an unsaturated resin containing nitrogen in the main chain with a vinyl monomer. Since this resin is prepared by radical polymerization, problems similar to those involved in the method disclosed in Japanese Patent Application Laid-Open Specification No. 101031/74 arise.
It is an object of the present invention to provide a toner composition for the electrophotography which can always give an image having a high quality with a small quantity of heat at a high copying speed while eliminating the necessity of maintenance.